首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   103605篇
  免费   5371篇
  国内免费   2678篇
林业   6534篇
农学   5525篇
基础科学   2267篇
  12146篇
综合类   25264篇
农作物   4808篇
水产渔业   4474篇
畜牧兽医   42496篇
园艺   2064篇
植物保护   6076篇
  2022年   1114篇
  2021年   1567篇
  2020年   1565篇
  2019年   1647篇
  2018年   1645篇
  2017年   2138篇
  2016年   1835篇
  2015年   1970篇
  2014年   2264篇
  2013年   4190篇
  2012年   3597篇
  2011年   4017篇
  2010年   3231篇
  2009年   3040篇
  2008年   3552篇
  2007年   3597篇
  2006年   3165篇
  2005年   2890篇
  2004年   2321篇
  2003年   2351篇
  2002年   2422篇
  2001年   2890篇
  2000年   2800篇
  1999年   2365篇
  1998年   1071篇
  1997年   1062篇
  1996年   891篇
  1995年   1086篇
  1994年   925篇
  1993年   883篇
  1992年   1690篇
  1991年   1738篇
  1990年   1792篇
  1989年   1768篇
  1988年   1626篇
  1987年   1531篇
  1986年   1550篇
  1985年   1528篇
  1984年   1272篇
  1983年   1160篇
  1979年   1261篇
  1978年   983篇
  1975年   915篇
  1974年   1203篇
  1973年   1243篇
  1972年   1256篇
  1971年   1184篇
  1970年   1102篇
  1969年   1003篇
  1967年   907篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
91.
92.
93.
94.
95.
96.
Crystalluria results from oversaturation of urine with crystallogenic substance. However, oversaturation may occur as a result of in vitro as well as in vivo events. The microscopic appearance of crystals only represents a tentative identification of their composition because variable conditions associated with their formation, growth, and dissolution may alter their appearance. Definitive identification is dependent on physical methods such as optical crystallography, x-ray diffraction, and electron microscopic analysis.  相似文献   
97.
The association of tumor-associated antigen (TAA) on the proliferation of BLV-infected lymphoblastoid B-cell lines (BL2M3 and BL312) was investigated. Flow cytometric analysis of the expression of TAA with monoclonal antibody (mAb) c143 showed high expression of TAA on the surfaces of BL2M3 and BL312 cells. A large amount of TAA was found in the culture supernatant of BL2M3 and BL312 cells as well as in the lysates of BL2M3 and BL312 cells. Culture supernatant but not lysates of BL2M3 and BL312 cells promoted the growth of either BL2M3 cells or BL312 cells. Furthermore, this growth promoting activity in culture supernatants of BL2M3 and BL312 cells was inhibited in a dose-dependent manner when cultured with mAb c143. These results suggested that TAA may be involved in the growth factor-mediated cell growth of bovine B-lymphoblastoid cell lines expressing TAA on their cell surface.  相似文献   
98.
Hyperchylomicronaemia was identified in a four-week-old Siamese kitten with lethargy, in-appetence, hindlimb ataxia and profound anaemia. The kitten was euthanased and at necropsy a thrombus was found occluding the caudal aorta. Two littermates later presented with lethargy, inappetence and hypertriglyceri-daemia which resolved after being weaned on to a low fat diet. A similar condition was subsequently diagnosed in a kitten born to the same sire but a different queen. The expression of hyperchylomicronaemia in two related litters was suggestive of an inherited, familial defect in the function of lipoprotein lipase (LPL). The activity of this enzyme was reduced in all three parents, the two recovered cases and two related, but apparently unaffected kittens, compared with a control group of unrelated cats belonging to the breeder. This reduction in activity was not attributable to defective activation of LPL by its serum cofactor apolipoprotein C-II or the presence in plasma of a factor that inhibited LPL. The gene that codes for LPL was examined by restriction fragment length polymorphism analysis using a human LPL cDNA probe. The results showed that the cat has a similar, but not identical, LPL gene to man. However, there were no differences in the restriction fragment patterns obtained from affected, unaffected and control animals.  相似文献   
99.
1. White blood cell responses of broilers, turkeys and ducks were examined at regular intervals after being subjected to various degrees of food restriction. 2. Restricted-fed broilers showed increases in heterophil and basophil numbers, together with a corresponding decrease in lymphocytes. The heterophil/lymphocyte ratio was raised. There were no differences between broiler strains. 3. After only one week of feeding restricted diets, heterophils were significantly raised in selected and unselected 2-week-old ducks. At 21 weeks of age, those ducks receiving 50% of food required to achieve their ad libitum-fed body weight had raised heterophils. 4. Ducks receiving food to achieve 25% of ad libitum-fed birds produced a marked basophilia, but no heterophilia. 5. After two weeks of food restriction, turkeys responded with significant heterophil/lymphocyte ratios following two degrees of restricted feeding. 6. It was concluded that in some poultry, a heterophilia may be the response to mild to moderate stress but a basophilia may result after severely stressing birds.  相似文献   
100.
本试验测定了pH、温度和EDTA对纯化的绵羊的瘤胃微生物胞外蛋白酶活性的影响,并用一已知氨基酸排列顺序的肽段(由15个氨基酸组成)作为底物,进一步测定了蛋白酶的酶切位点。结果表明,(1)纯化蛋白酶的最适pH在6.0~6.5之间,最适温度为40℃左右。(2)不同浓度的EDTA(浓度分别为1,10,25,50,75和100mM)对蛋白酶活性没有影响,因而该酶属于丝氨酸蛋白酶类。(3)纯化蛋白酶是一种内肽酶,其酶切位点在组氨酸—酪氨酸(?)、天冬氨酸—丙氨酸(?)、亮氨酸—赖氨酸(?)和(或)缬氨酸—赖氨酸(?)相连的肽键。(4)胰蛋白酶和纯化的蛋白酶虽同属丝氨酸蛋白酶类,但前者对底物具有高度的专一性,后者对底物的专一性不强,这是瘤胃内饲料蛋白质强烈降解的主要原因之  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号